In cellular systems such as, for example a Wideband Division Multiple Access (WCDMA) or Global System for Mobile communication (GSM) cellular system power control is used to set the received signal at an appropriate level for reception, while minimizing interference to other users. A mobile station (MS) may send a power control commands to a base station (BS) to control the power of the signal it receives.
In WCDMA cellular system a MS is capable of operating in a compress mode (CM). Whilst operating in the CM mode, the communication between the BS and the MS is compressed in time, leaving time gaps which are used by the MS to search for base stations that operate in frequencies other than the current operating frequency of the MS.
Operation in CM mode may be applicable by the BS over a downlink (DL) and/or by the MS over an uplink (UL), separately. In operation in CM mode over the DL, the BS compresses the information it sends over the DL, from a whole data frame to a portion of the data frame in order to create gaps in the DL transmission. For example, the BS may shorten the duration of each symbol without changing the number of symbols in a frame, which is known in the art as spreading factor reduction. The gaps in the DL transmission may be used by the MS to perform measurements on different frequencies.
In a WCDMA cellular system, each BS uses a unique primary scrambling code (PSC) to spread its transmission over the whole frequency bandwidth (BW). During compressed frames, the BS may use an alternate scrambling code (ASC) on the portion of the transmission of the MS which is in the CM. On the other portion of the transmission the BS may use the PSC.
According to a present art of WCDMA cellular system, the WCDMA downlink power control loop is based on two loops: an outer loop and an inner loop. The inner loop measures a dedicated channel (DCH) signal to interference ratio (SIR) in each slot and compares the DCH SIR to a target SIR. If the measured DCH SIR is higher than the target SIR than a command to decrease a DCH power is sent to the BS. Otherwise, a command to increase the DCH power is sent to the BS. The outer loop sets the target SIR to achieve a quality of service value (QoS) given by a target block error rate (BLER).
In CM, the MS may search in a predefined time window (e.g., the gap) on the DCH, the existence of either GSM or WCDMA networks on frequencies different than its operation frequency. A WCDMA BS may transmit in the predefined time window, compressed frames scrambled by an ASC while non-compressed frames are scrambled by a PSC. It should be understood that the PSC is not orthogonal to the ASC. This may cause a large SIR difference between uncompressed frames and compressed frames of the DCH. Thus, the difference between the frames may not be compensated by the inner loop within the duration of the compressed frame. The SIR difference may force the outer loop to increases the SIR target in order to maintain the required QoS and to cause a large increase in the average DCH signal power. This will increase the interference to other users and reduce the capacity of the WCDMA network.
It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements.